Data Visualization with R
More on the overall appearance of plot
require(ggplot2)
require(ggthemes)
require(xtable)
require(qdata)
data(bands)The overall appearance of plots is controlled by the theming system, an important component of ggplot2 grammar. We learned about the theming system in the previous chapters, in particular in “Legend Customization”, “Axes Customization” and “Facet Customization”, where we saw how themes give us the control of non-data elements of the plot like fonts, ticks, panel strip, legend keys, ..
Let us deeepen the theming system structure.
ggplot2 has a default theme, called theme_grey(), with a light grey background and white gridlines.
A theme, or theme function defines the settings of a collection of theme elements for the purpose of creating a specific style of graphics production.
Let us see an example, considering the relationship between humidity and viscosity by pressure type in bands dataset:
pl <- ggplot(data = bands, mapping = aes(x= humidity, y = viscosity, colour =press_type)) +
geom_point()
pl
In particular, a theme function is composed of:
theme elements, which refer to individual attributes of a graphic that are independent of the data and that you can control, such as font size, axis ticks, appearance of grid lines or background color of a legend;
theme element functions, which enables you to modify the settings of certain theme elements. In particular, each theme element is associated with an element function, which describes the visual properties of that element.
Each theme element has a default value that can be locally modified in a specific ggplot object by using a theme() function.
Suppose we want to modify the colour of axis lines:
pl +
theme(axis.line.x = element_line(colour = "black"),
axis.line.y = element_line(colour = "black"))
In particular, axis.line.x and axis.line.y are the theme elements that control the appearance of x and y axis respectively and element_line() is the theme element function that allows us to modify the theme elements.
Moreover, if you don’ like theme_grey() you can decide to totally replace it by setting another theme function (complete themes). Let us set theme_bw(), a theme with white background and thin grey grid lines:
pl +
theme_bw()
Understanding how dealing with theme element functions and theme elements is very important in plot customization phase.
The most important theme element functions are:
element_text(): controls the drawing of labels and headings.
The table below lists its arguments and their corresponding default values:
| Argument | Description | Default Value |
|---|---|---|
| family | font family | ’’ |
| face | font face | ‘plain’ |
| colour | font color | ‘black’ |
| size | font size (pts) | 10 |
| hjust | horizontal justification | 0.5 |
| vjust | vertical justification | 0.5 |
| angle | text angle | 0 |
| lineheight | line height | 1.1 |
element_line(): draws lines and segments such as graphics region boundaries, axis tick marks and grid lines.
The table below lists its arguments and their corresponding default values:
| Argument | Description | Default Value |
|---|---|---|
| colour | line color | ‘black’ |
| size | line thickness | 0.5 |
| linetype | type of line | 1 |
element_rect(): draws rectangles. It is mostly used for background elements and legend keys.
The table below lists its arguments and their corresponding default values:
| Argument | Description | Default Value |
|---|---|---|
| fill | fill color | NA (none) |
| colour | border color | ‘black’ |
| size | thickness of border line | 0.5 |
| linetype | type of border line | 1 (solid) |
element_blank(): draws nothing. This function has no arguments.
There are around 40 unique theme elements that controls the appearance of the plots. They can be roughly grouped into five categories: plot, axis, legend, panel and facet.
Let us schematize them:
- plot theme elements
| Element | Setter | Description |
|---|---|---|
| plot.background | element_rect() | plot background |
| plot.title | element_text() | plot title |
| plot.margin | unit() | margins around plot |
Let us see an example:
pl +
labs(title = "Plot title") +
theme(plot.title = element_text(size = 20, vjust = 2),
plot.background = element_rect(
fill = "lightblue",
colour = "black",
size = 2,
linetype = "longdash"),
plot.margin = unit(c(1, 1, 1, 1), "in"))
- axis theme elements
| Element | Setter | Description |
|---|---|---|
| axis.line | element_line() | line parallel to axis (hidden in default theme) |
| axis.text | element_text() | tick labels |
| axis.text.x | element_text() | x-axis tick labels |
| axis.text.y | element_text() | y-axis tick labels |
| axis.title | element_text() | axis titles |
| axis.title.x | element_text() | x-axis title |
| axis.title.y | element_text() | y-axis title |
| axis.ticks | element_line() | axis tick marks |
| axis.ticks.length | unit() | length of tick marks |
| axis.ticks.margin | unit() | width of axis tick margin |
Let us see an example:
pl +
theme(
axis.line.x = element_line(colour = "red", size = 2),
axis.line.y = element_line(colour = "orange", linetype = "dashed"),
axis.text = element_text(color = "blue", size = 15, face = "italic"),
axis.text.y = element_text(angle = 90, size = rel(0.7), hjust = 0),
axis.ticks = element_line(colour = "violet"),
axis.ticks.x = element_line(size = rel(2)),
axis.title = element_text(size = 20, color = "orangered")
)
- legend theme elements
| Element | Setter | Description |
|---|---|---|
| legend.background | element_rect() | legend background |
| legend.key | element_rect() | background of legend keys |
| legend.key.size | unit() | |
| legend.key.height | unit() | legend key height |
| legend.key.width | unit() | legend key width |
| legend.margin | unit() | legend margin |
| legend.text | element_text() | legend labels |
| legend.text.align | numeric | legend label alignment |
| legend.title | element_text() | legend name |
| legend.title.align | numeric | legend name alignment |
| legend.position | ‘left’, ‘right’, ‘bottom’ ‘, ’top’ | position of legend |
| legend.direction | numeric | direction of legend keys |
| legend.justification | numeric | justification of legend |
| legend.box | numeric | position of multiple legend boxes |
Let us see an example:
pl +
theme(
legend.position = "top",
legend.box = "horizontal",
legend.background = element_rect(
fill = "lemonchiffon",
color = "black",
size = 1,
linetype = "longdash"
),
legend.key = element_rect(fill = "lemonchiffon", color = "magenta"),
legend.key.width = unit(0.5, "in"),
legend.key.height = unit(0.2, "in"),
legend.text = element_text(size = 8),
legend.title = element_text(face = "bold", size = 10, colour = "magenta"))
- panel theme elements
| Element | Setter | Description |
|---|---|---|
| panel.background | element_rect() | background of graphics region |
| panel.border | element_rect() | border of graphics region |
| panel.grid.major | element_line() | major grid lines |
| panel.grid.major.x | element_line() | vertical major grid lines height |
| panel.grid.major.y | element_line() | horizontal major grid lines |
| panel.grid.minor | element_line() | minor grid lines |
| panel.grid.minor.x | element_text() | vertical minor grid lines |
| panel.grid.minor.y | element_line | horizontal minor grid lines |
| panel.margin | numeric | margin between facets |
| aspect.ratio | numeric | plot aspect ratio |
Let us see an example:
pl +
theme(
panel.background = element_rect(fill = "navy", color = "orange", size = 2),
panel.border = element_rect(fill = NA, colour = "orange", size = 2),
panel.grid.major = element_line(color = "gray60", size = 0.8),
panel.grid.major.x = element_blank())
- facet theme elements
| Element | Setter | Description |
|---|---|---|
| strip.background | element_rect() | background of panel strips |
| strip.text | element_text() | strip text |
| strip.text.x | element_text() | horizontal strip text |
| strip.text.y | element_text() | vertical strip text |
Let us see an example:
ggplot(data = bands, mapping = aes(x= humidity, y = viscosity)) +
geom_point() + facet_grid(band_type ~ press_type) +
theme(
strip.background = element_rect(fill = "#4bb8b6", color = "#265665", size = 2),
strip.text = element_text(face = "italic", size = 15, colour = "#CC1800"),
strip.text.y = element_text(face = "bold")
)
We have already learn about most of these ggplot2 theme elements in the previous chapters, in particular: axis theme elements, legend theme elements and facet theme elements. But what’s about plot theme elements and panels theme elements?
In the following paragraphs we will see how to handle with the most common questions on plot, panels and theme customization.
Add the title to a plot and change its appearance
As we saw in “Creating a Boxplot” chapter, title can be added by using ggtitle() or labs function in this way:
pl +
ggtitle("Scatterplot of humidity vs viscosity \n by Pressure type")
pl +
labs(title = "Scatterplot of humidity vs viscosity \n by Pressure type")The previous two commands produces the same result:
"\n" is used to break the lines.
Title appearance can be changes by setting plot.title argument of theme() function:
pl +
ggtitle("Scatterplot of humidity vs viscosity \n by Pressure type") +
theme(plot.title=element_text(size=rel(2), lineheight=.9, family="Times", face="bold.italic", colour="red"))
Modify the appearance of plotting area
If you want to change the appearance of plotting area you have to set panel_xxx arguments of theme() function.
Change the background of plotting area
panel.background element allows us to modify the background of graphical region and panel.border to modify the border of graphical region. Both panel.background and panel.border are modified by using element_rect() function.
pl +
theme(
panel.background = element_rect(fill="lightblue"),
panel.border = element_rect(colour="blue", fill=NA, size=2)
)
Pay attention to fill argument for panel.border: you have to specify a blank fill (setting it to NA) for not covering panels.
Customize and remove grid lines
panel.grid.major and panel.grid.minor arguments allows us to customize grid lines by using element_line() function:
pl +
theme(
panel.grid.major = element_line(colour="red"),
panel.grid.minor = element_line(colour="red", linetype="dashed", size=0.2)
)
If you want to remove grid lines you have to set panel.grid.major and/or panel.grid.minor equal to element_blank():
pl +
theme(
panel.grid.major = element_blank(),
panel.grid.minor = element_blank()
)
It’s possible to hide just the vertical or horizontal grid lines with panel.grid.major.x, panel.grid.major.y, panel.grid.minor.x and panel.grid.minor.y arguments of theme() function:
pl +
theme(
panel.grid.major.x = element_blank(), # remove horizontal grid major lines
panel.grid.minor.y = element_blank() # remove vertical grid minor lines
)
Modify the plot appearance outside graphical area
If you want to modify plot appearance outside graphical area you have to set plot_xxx arguments of theme() function.
Change the background outside plotting area
To modify the background set plot.background element of theme() function:
pl +
theme(
plot.background = element_rect(fill = "green")
)
Change the margins around graphical area
To modify margins around graphical area set plot.margin element of theme() function:
pl +
theme(
plot.margin = unit(c(2,2,2,2), "cm")
)
unit() is a function that creates a grid unit object of the correct length to use for setting margins.
Change the default theme
To change theme function you have to overwrite the defaut one. Other theme functions are provided by ggplot2 and ggthemes packages.
ggplot2 provides these theme functions:
theme_bw(): a variation ontheme_grey()that uses a white background and thin grey grid linestheme_linedraw(): a theme with only black lines of various widths on white backgrounds, reminiscent of a line drawing
theme_light(): similar totheme_linedraw()but with ligth grey lines and axes, to direct more attention towards the datatheme_dark(): the dark cousin oftheme_light(), with similar line sizes but a dark background. Useful to make thin coloured lines pop out:theme_minimal(): a minimalistic theme with no background annotationstheme_classic(): A classic-looking theme, with x and y axis lines and no gridlines. Inggplot2.1.0 axes are not visible because of a bug in the functiontheme_void(): a completely empty theme
pl1 <- pl + theme_bw() + ggtitle("theme_bw()")
pl2 <- pl + theme_linedraw() + ggtitle("theme_linedraw()")
pl3 <- pl + theme_light() + ggtitle("theme_light()")
pl4 <- pl + theme_dark() + ggtitle("theme_dark()")
pl5 <- pl + theme_minimal() + ggtitle("theme_minimal()")
pl6 <- pl + theme_classic() + ggtitle("theme_classic()")
pl7 <- pl + theme_void() + ggtitle("theme_void()")
gridExtra::grid.arrange(pl1, pl2, pl3, pl4, pl5, pl6, pl7, ncol=2)
ggthemes package provides lots of theme functions. Let us see the most used:
pl8 <- pl + theme_tufte() + ggtitle("theme_tufte()") # a minimal ink theme based on Tufte’s The Visual Display of Quantitative Information
pl9 <- pl + theme_solarized() + ggtitle("theme_solarized()") # a theme using the solarized color palette
pl10 <- pl + theme_excel() + ggtitle("theme_excel()") # a theme replicating the classic gray charts in Excel
pl11 <- pl + theme_few() + ggtitle("theme_few()") # theme from Stephen Few’s “Practical Rules for Using Color in Charts”
pl12 <- pl + theme_economist() + ggtitle("theme_economist()") # a theme based on the plots in the The Economist magazine
pl13 <- pl + theme_stata() + ggtitle("theme_stata()") # themes based on Stata graph schemes
pl14 <- pl + theme_wsj() + ggtitle("theme_wsj()") # a theme based on the plots in the The Wall Street Journa
gridExtra::grid.arrange(pl8, pl9, pl10, pl11, ncol=2)
pl12
pl13
pl14
Chang the default theme for more than one plot
If you want to change the default theme (theme_grey()) for all plots generated in the current R session, you can use theme_set() function. For example, if you want to use white background for all plots run:
theme_set(theme_bw())Creating your own theme
You can create your own theme by adding elements to an existing theme:
mytheme <- theme_bw() +
theme(text = element_text(colour="red"),
axis.title = element_text(size = rel(1.25)))pl +
mytheme